View the Primary Results tab. This lists the content to which external users have access under the root site of your tenancy. Ignore the results from the site to which they were invited (https://contoso.sharepoint.com/sites/externalusertest).

Repeat the query by using the following Query Text to review access to OneDrive content:

path:https://contoso-my.sharepoint.com

The results will include access to some system ASPX pages that have no content. Those pages can be ignored.

Then, you can investigate any results individually to determine whether they are permissioned correctly.

Reference

For more information about how to govern access of external users in Office 365, refer to the following Microsoft Help article: